Fragile X syndrome (FXS) is the leading cause of inherited intellectual disability, affecting as many as 1 in 2500 people. The syndrome is caused by an expansion of a repetitive sequence of nucleotides in the FMR1 gene on the X chromosome, which leads to a dramatic reduction in a protein (FMRP) essential for experience- dependent synaptic development. The proposed project is designed to examine the language development of males with FXS during adolescence and young adulthood, the factors that affect that development, the consequences of language impairments for independent functioning in adulthood, and the specificity of the language profile observed. The project will extend the follow up of an already well-characterized cohort of males with FXS as well recruiting new participants. The project involves an accelerated longitudinal design, with four annual assessments and participants entering between the ages of 15 and 22 years. Four specific aims will be addressed. (1) The developmental trajectory of important, theoretically motivated, components of language will be described in males with FXS relative to an already characterized typically developing comparison sample of boys of comparable nonverbal mental age. The battery of measures employed will include standardized tests, expressive language samples, and laboratory-based measures. These measures will assess competence in vocabulary, syntax, and pragmatics, as well as atypical language behavior (i.e., perseveration). (2) Potentially important determinants of within-syndrome variation in language development will be investigated. The focus will be on core psychological and behavioral features of FXS (i.e., working memory, autism symptoms, social avoidance, and anxiety), as well as genetic factors (i.e., FMRP expression) and environmental factors (e.g., family emotional climate). (3) The impact of linguistic impairments on adult outcomes reflective of independence participation in the community will be investigated. (4) We will identify those aspects of the language development profile of FXS that are similar (or dissimilar) to idiopathic autism, thereby yielding insights into syndrome specificity and the underlying pathology. This last aim will be addressed by collecting comparable data on a cohort of age- and nonverbal mental age-matched males with idiopathic autism. The primary analytical tool will be hierarchical linear modeling, although other techniques, such as multiple regression, also will be used.